Bound lipid in the tissues of mammal and insect: a new histochemical method.

'Bound' lipids persist in ester wax sections of tissues fixed in glutaraldehyde. They do not take up Sudan black B. They are unmasked by dilute sodium hypochlorite and then stain readily with the Sudan dye and are extractable in warm chloroform and methanol. Bound lipids have been studied in tissues from the mouse and from the insects Rliodnius, Periplaneta and Calliphora. They are so widely distributed that Sudan staining alone gives an informative histological picture. They occur in nucleoli and the nuclear envelope, in the nuclear matrix and in the cytoplasm (especially when rich in endoplasmic reticulum), in plasma membranes and their invaginations and evaginations (microvilli), in septate desmosomes, and particularly in mitochondria, dictyosomes and myelin. In erythrocytes they are not only concentrated in the surface membrane but are distributed throughout the interior of the cell. In muscle fibrils (papillary muscle of the ventricle and fibrilIar flight muscle of Calliphora) bound lipid occurs in small amounts in the dark regions of the unstained muscle (' A '-bands) but is normally absent from the ' Z '-line and from the ' I 'bands. In highly contracted muscle this relation is reversed: lipid is concentrated around the ' Z '-line. The elastic layers and fibrils in arteries are rich in bound lipid. Bound lipids are presumably lipoproteins. Purified lipoprotein from egg yolk examined histochemically has the same properties.